1. Field of the Invention
The invention relates to optical fiber. More particularly, the invention relates to fabricating optical fibers having improved transmission characteristics over the wavelength region 700-1600 nanometers.
2. Description of the Related Art
Optical fibers typically are fabricated by heating and drawing a portion of an optical preform usually comprising a solid glass rod with a refractive glass core surrounded by a protective glass cladding having a lower refractive index than that of the core. The glass fiber is coated with one or more layers of protective coating materials cured, e.g., by radiation.
Conventionally, several processes exist for fabricating optical preforms, including modified chemical vapor deposition (MCVD), vapor axial deposition (VAD) and outside vapor deposition (OVD). In conventional VAD and OVD processes, layers of glass particles or xe2x80x9csootxe2x80x9d are deposited on the end surface or the outside surface, respectively, of a starter rod. The deposited soot layers then are dried or dehydrated, e.g., in a chlorine or fluorine-containing atmosphere, and sintered or consolidated to form a solid preform core rod.
Once the preform core rod is formed, optical fiber is drawn directly therefrom or, alternatively, one or more overclad layers are formed thereon prior to drawing optical fiber therefrom. The overclad layers are formed on the preform core rod, e.g., by a soot deposition technique similar to that used in forming the preform core rod. Alternatively, the overclad layers are formed by collapsing a silica-based tube or sleeve around the preform core rod. Such process typically is referred to as the Rod-In-Tube (RIT) process. See, e.g., U.S. Pat. No. 4,820,322, which is co-owned with this application, and hereby is incorporated by reference herein.
The transmission characteristics of optical fiber vary based on a number of factors, including, e.g., scattering such as Rayleigh scattering, fiber bending and absorption such as hydroxyl-ion (OH) absorption. OH absorption, or xe2x80x9cwaterxe2x80x9d absorption, is of particular interest because it reduces useful bandwidth in an otherwise relatively low transmission loss wavelength region from 700-1600 nanometers (nm), i.e., the wavelength region in which many current optical systems operate.
OH absorption, which is due to vibrational overtones of hydroxyl-ions in the fiber, typically causes three loss peaks within the 700-1600 nm region: 950 nm, 1240 nm, and 1385 nm. It has been desirable to reduce these water loss peaks, particularly the water loss peak centered around 1385 nm, since concentrations of OH as low as 1 part per million (ppm) can cause losses as great as 65 dB/km at 1385 nm in single mode fibers. Furthermore, reduction of the water loss peak at 1385 nm effectively provides an uninterrupted region of relatively low transmission loss from 1200-1600 nm. The wavelength region from 1200-1600 nm is of special interest because of the abundant availability of light sources such as Indium Phosphide (InP)-based sources. See, e.g., U.S. Pat. No. 6,131,415, which is co-owned with this application and assigned to the assignee of this application.
Conventional techniques for reducing the adverse affects of water loss include exchanging a portion of the hydrogen atoms with deuterium atoms in a high temperature (e.g., approximately 1000xc2x0 Celsius) OD to OH exchange reaction. See, e.g., U.S. Pat. No. 4,445,918; U.S. Pat. No. 4,583,997; and U.S. Pat. No. 4,389,230, in which deuterium is provided during various stages of the manufacture of optical fiber preforms. Also, see U.S. Pat. No. 4,685,945, in which optical fiber preforms are exposed to deuterium in combination with high intensity light to reduce OH defects.
It should be noted that deuterium also has been used to effect refractive index changes in glass bodies, including optical fibers. See, e.g., U.S. Pat. No. 5,930,420; U.S. Pat. No. 5,500,031; and U.S. Pat. No. 4,515,612, all of which are co-owned with this application and assigned to the assignee of this application.
Another type of absorption loss that is sought to be reduced or eliminated is aging loss including the hydrogen aging loss that occurs during the lifetime of the fiber. Such losses are thought to occur because of the chemical reaction between various defects in the optical fiber and hydrogen in the optical fiber environment, e.g., within an optical fiber cable environment. Such defects include, e.g., germanium (Ge) defects and silicon (Si) defects introduced into the optical fiber during its fabrication. The chemical reactions are made possible, e.g., by the ability of hydrogen to diffuse readily into the optical fiber.
It would be desirable to have available a method for making optical fibers, including single mode optical fibers, that further reduces aging or hydrogen aging loss over the life of the fiber and optical fiber systems including such optical fibers.
The invention is embodied in a method for making optical fiber having reduced aging or hydrogen aging loss over the life of the fiber and optical fiber systems including such optical fibers. Embodiments of the invention provide improved silicon-oxygen stoichiometry in fiber manufacturing environments to reduce the amount of Si defects in an optical fiber preform, combined with subsequent deuterium exposure of the fiber drawn from the preform to reduce the likelihood of having atomic defects in the optical fiber that, over time, attract and bond with hydrogen atoms to form molecules that contribute to increased water absorption loss. Such improved silicon-oxygen stoichiometry has neither excessive oxygen atoms, which reduces the number of oxygen-rich defects (Sixe2x80x94Oxe2x80x94Oxe2x80x94Si defects) that are formed and subsequently trapped in the silica glass, nor is a deficiency in oxygen atoms, which reduces the number of oxygen-deficient defects (Sixe2x80x94Si defects) formed. Also, during the subsequent deuterium exposure, deuterium atoms react with Si defects such as Sixe2x80x94Oxe2x80xa2defects and Sixe2x80xa2defects to form SiOD or SiD, respectively, thus reducing the amount of Si defects in the fiber and consequently the possible amount of reactions between Si defects and hydrogen, which reactions typically cause SiOH and SiH losses in the fiber.
Methods for making optical fibers according to embodiments of the invention include the steps of manufacturing an optical fiber preform, drawing fiber from the preform, and exposing the drawn fiber to deuterium, e.g., by exposing the drawn fiber to a deuterium atmosphere having, e.g., a partial pressure of approximately 0.01 atmospheres of deuterium at room temperature for approximately 6 days, or, alternatively, a partial pressure of approximately 0.05 atmospheres of deuterium at room temperature for approximately 1.5 days. The fiber manufacturing steps include forming a glass core rod, dehydrating the glass core rod, consolidating the glass core rod, and forming an overclad region around the glass core rod. According to embodiments of the invention, dehydration occurs in an atmosphere containing oxygen or oxygen and one or more chlorine-containing gases, fluorine-containing gases and/or carbon monoxide (CO), with the partial pressure of the gases established and/or adjusted to provide an environment that is neither oxygen-rich or oxygen-deficient. Alternatively, the overclad region formation step occurs in a similar atmosphere, with the partial pressure of the gases established for improved silicon-oxygen stoichiometry.
Optical fiber made by methods according to embodiments of the invention have improved transmission characteristics. For example, the inventive optical fibers have transmission loss at 1385 nanometers that is less than 0.33 dB/km and the loss increase thereafter is less than 0.04 dB/km.